This invention relates to pulse code modulation (PCM) frame slip detection and, more particularly, to an arrangement for detecting the occurrence of a PCM frame slip solely within a single voice channel in contrast to the customary detection of frame slip by comparision of PCM bit stream rates.
In the most common PCM transmission system, each voiceband customer signal is sampled at a frame rate of 8,000 times per second and each sample is encoded into eight bits. These eight-bit samples of 24 customer channels plus a single frame bit are combined into a 193 bit frame format lasting 1/8,000 second or 125 microseconds. The resulting bit rate thus is 193.times.8,000=1,544,000 bits per second or, equivalently, 1.544 megabits per second (Mb/s). Although the details of the frame structure may be different for various PCM systems throughout the world, the channel frame rate of 8,000 per second is observed almost universally, i.e., the incidence of other than 125 microsecond frame duration is almost inconsequential.
Whereas a connection with digital carriers and multiple digital switches all of which have perfectly synchronized bit streams will show no evidence of frame slip, (i.e., repetition or deletion of a full 125 microsecond frame of data), it has been found that a slight lack of synchrony (and the slips which result) is not detrimental to voice traffic in a PCM channel. Moreover, elimination of synchronization equipment represents an element of cost saving at no apparent penalty to the typical voice customer. However, the effect of even a single frame slip on the transmission of digital data over the voice channel by data modems may be devastating insofar as data errors are produced. To see how this arises, it is necessary to note how asynchrony between the received PCM stream and the local PCM receiver are handled.
If we assume that both the incoming PCM rate and the PCM receiver clock rate are within the established standards, they will differ by only a slight amount with one running relatively faster than the other. Should the incoming PCM signal rate exceed the receiver clock rate, at periodic intervals related to the difference in the rates a whole 125 microsecond frame will have to be deleted in the channel in order to coordinate to incoming signals with the clock at the receiver. On the other hand, should the receiver clock rate exceed the incoming PCM signal rate, at periodic intervals there will be no new and valid PCM sample to be offered to the receiver. At those times, it is again impossible to materialize a proper input because of a frame slip. In those circumstances, it is customary for the PCM receiver to retain and repeat the previous frame's data for use as the missing data. This results in an apparent time stretch of 125 microseconds. As said before, the effect of such "controlled" slips, as they are known, is of no consequence on voice traffic but may be very serious to data traffic. Such a slip-induced 125 microsecond shift in the sampling times in the modem receiver will cause numerous errors until the timing circuit recovers.
Various techniques have been used to identify a slip at the 1.544 Mb/s pulse rate, using external test equipment or equipment which receives the 1.544 Mb/s pulse stream and compares it to a reference clock rate, such as a digital switch clock rate. None of these conventional techniques, however, allows a customer whose signal is confined to one of the voice channels, and who typically does not have access to the 1.544 Mb/s bit stream, to identify that a controlled slip has occurred. Moreover, this controlled slip may occur in an interior portion of the total transmission system so that the end user has no means available to detect slip in the PCM portion.